The invention relates to apparatus and process for the biological treatment of liquid wastes employing fluidized beds. In particular, it is directed to a process for removing ammonia nitrogen from waste water.
Untreated municipal wastes generally contain from 20 to 50 milligrams of nitrogen per liter, mostly in the form of ammonia and organic nitrogen. The serious detrimental environmental effects of these compounds had not been fully realized until the last decade. With the large amounts of fixed nitrogen in the form of ammonia and other compounds that are being introduced into the biosphere by the large scale use of synthetic fertilizers, and with the demands man makes on his environment owing to population congestion, there definitely appears to be an imbalance developing in our ecological system that may have long range consequences for future generations. The presence of such nutrients in natural waters causes fertilization and vegetative growth in the form of algal blooms. Such blooms often result in accelerated eutrophication.
Conventional methods of municipal sewage treatment, chiefly activated sludge and trickling filtration are designed to remove solids and oxygen demanding organic material from the waste water. During these treatment processes some of the organic nitrogen is converted into the ammonia form. Chemical and physical methods such as chlorination and ion exchange have been tried in small scale experiments to remove these ammonia compounds from the waste water, but costs have been too prohibitive to attempt these methods of treatment in large scale installations. Although ammonia stripping is economically feasible, it suffers the disadvantages of poor operation or shutdown in winter and the introduction of ammonia into the atmosphere.
Biological methods of treatment have been most frequently used to remove ammonia in typical large installations. Oxidation of ammonia nitrogen to nitrate nitrogen can be accomplished in an activated sludge treatment plant by increasing aeration time in the plant from 3 - 6 hours to 10 or more hours. This requires the use of large aeration basins and is often inefficient because of difficulties in controlling the system. This oxidation of ammonia to nitrate, termed nitrification, can also be accomplished by aerating the effluent from the activated sludge treatment process in a separate aeration basin. This facilitates control of the nitrification process but requires additional aeration basins with an additional aeration time of 3 to 6 hours.
Certain experimental nitrification processes have employed the use of up-flow columns or beds. Such packed beds tend to become clogged as solids in the waste water are filtered out and further as attached biota undergo uncontrolled growth on the stone media. Such blockage causes insurmountable head losses. These losses must be relieved by frequent and impractical back washing of the bed. Also, detention times in excess of one hour are required.
An example of a prior art system is described in the publication by, St. Amant, P.P. and McCarty, P.L. "Treatment of High Nitrate Waters," JOURNAL OF AMERICAN WATER WORKS ASSOCIATION, pp. 659-669, 1969. This publication is concerned with an up-flow denitrification system, which is basically a packed bed of one inch stone, as compared to the present application which is concerned with a fluidized bed of small particles. Hence, the operating parameters and results are completely different. Another example of a prior art system is described in the publication by, Weber, W. J. Jr. and Morris, J. C. "Kinetics of Adsorption in Columns of Fluidized Media," JOURNAL OF AMERICAN WATER WORKS ASSOCIATION, pp. 425, 430, 443, 1965. This publication teaches the use of an expanded bed column for a physical adsorption process, i.e., the adsorption of organic carbon by porous adsorbent activated carbon particles. The process of Weber et al does not rely upon the use of biological action, as is the case in the present application.
Still another example of prior art systems is the Savage, U.S. Pat. No. 3,709,364 issued in January 1973. The process described in this patent is essentially a "deep bed filter" which employs a down-flow system. With this type of system, as the spaces between the particles become plugged with solid wastes, great head losses result. Savage recognized this problem and provided means for intermittent back-washing to agitate this filter media and remove suspended solids collected on it. Thus, the Savage system was predicated on different principles and employed different parameters as compared to the present application.
Other related patents and publications in this art include the following:
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